As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
N1'S FLUCTUATING AT IDLE BOTH ENGINES 23.3-26.5 ACCOMPANIED BY INTERMITTENT VIBRATION IN COCKPIT THAT DOES NOT APPEAR ON VIBRATION GAUGES. ALL OTHER EICAS GAUGES WITHIN LIMITS. AIRCRAFT BACK TO BLOCK AND GROUNDED BY MAINTENANCE. MAINTENANCE REMOVED AND REPLACED LH N1 TACH. TRANSMITTER IAW B767 AMM 77-12-01. ACCOMPLISHED N2 INDICATION OPS CHECK ON BOTH ENGINES IAW B767 AMM 77-12-00 OPS CHECK GOOD. AIRCRAFT RETURNED TO SERVICE.